1. Technical Field
The invention relates to an image processing apparatus, an image processing method, a computer-readable medium and a computer data signal.
2. Related Art
When an image is output, the density of the image is expressed by changing dot sizes or dot densities according to values of respective pixels. Processing of changing the values of the pixels to the dot sizes or the dot densities is called screen processing. Also, the changing to the dot sizes is referred to as AM screen processing, and the changing to the dot densities is referred to as FM screen processing.
When a color image is output, the screen processing is performed for respective color components. In the AM screen processing, if the screen processing for the respective color components are performed in a fixed direction, when dots of all the color components overlap with each other, there would arise no problem; but when dots of the respective color components don't overlap with each other, apparent color changes, which results in color unevenness. Therefore, dot arrangement directions of the respective color components are shifted. Thereby, portions in which dots of the respective color components don't overlap with each other and portions in which they overlap with each other are mixed, so that the color unevenness is reduced as a whole. In this case, the dot arrangement direction is referred to as a screen angle.
When an image consists of five color components including one spot color, it has been known that: an angle range of from 0° to 90° is equally divided into five, and five screen angles of 18°, 36°, 54°, 72° and 90° are assigned to the respective color components.
Since the FM screen processing has no periodicity unlike the AM screen processing, it is hard that interference fringes occur. However, the FM screen processing has such a defect that granularity is poor and color stability is poor, compared with the AM screen processing. Therefore, a color printer, an electrographic image output apparatus, and the like frequently adopt the AM screen processing.